Got an error message for part of a rule file that I can't decipher:
"Recursive rule detected at line ..."
pointing to the "=>" in
org.openide.filesystems.Repository.getDefault().findResource($r)
=>
org.openide.filesystems.Repository.getDefault().getDefaultFileSystem().findResource($r)
::
$r instanceof java.lang.String;
I can't see how this could be interpreted as recursive but Jackpot doesn't seem
to like it.

I don't see how it's recursive either, but there may be another issue. What you
are really seeing is that the method frame depth for the transformer exceeded a
built-in max, which is normally caused by recursive rules (the error points to
the rule being matched in the offending frame). I'll need to break at the
exception creation to see what the problem is, as the rule looks fine.
Is this rule part of a larger file?